The epoxy cresol novolac (ECN) resins have become important items of commerce with a myriad of users based on their highly desirable physical properties after curing. These ECN resins with a multiplicity of glycidyl ether groups in each molecule lead to cured resins with high crosslink density and great rigidity. Such properties are particularly important in the electronic industry for circuit boards and other end-uses.
Epoxy novolac resins are prepared by reacting a novolac resin with excess epichlorohydrin, based on the phenolic hydroxyl group of the novolac resin, in a strongly alkaline medium, such as aqueous sodium hydroxide, to give first the corresponding chlorohydrin ether intermediate which is then dehydrochlorinated to form the desired glycidyl ether or epoxy novolac resin product.
The process is illustrated schematically below with a cresol novolac resin. ##STR1##
Ideally all the chlorohydrin ether intermediate is converted by dehydrochlorination into the epoxy novolac resin leaving "no" reactive chlorine content in the final product. Such chlorine if still present would be potentially reactive in the presence of moisture or chemical reagents and is designated as hydrolyzable chlorine. The presence of such hydrolyzable chlorine would clearly have detrimental and deleterious effects on the final properties of the cured epoxy novolac resins particularly in the electrical and electronics industries where low chlorine and alkali metal ion contents are needed for the flawless encapsulating of electronic devices. The cured epoxy novolac resins must have low electric conductivity to minimize the electrical failure of electronic devices such as memory chips and semiconductors. Commercial ECN resins do not always meet these purity requirements.
The use of potassium tert-butylate to effect the dehydrohalogenation of alpha-halocarboxylic acids has been reported by J. Cason et al, J. Org. Chem. 18, 850 (1953).
The use of such alkali metal alkoxides with the dehydrohalogenation of epoxy resins, particularly EPN or ECN resins, is unknown.